This invention relates to a method for producing high-quality carbon fibers rapidly and efficiently from acrylic fibers.
More particularly, it relates to a method for producing high strength carbon fibers without causing self adhesion or fusion bonding between them at a calcination step at high temperatures.
Many proposals have been made for making high-strength and high-elasticity carbon fibers on an industrial scale since it has been found that acrylic fibers are useful for the making of carbon fibers. Especially, when carbon fibers are used as reinforcing materials for composite materials, a high tensile strength is required and further, stable performance is desired not only as single filaments, but as fiber tows.
For satisfying these demands, it is necessary to carry out under optimum conditions the calcination process for converting the precursor acrylonitrile fiber tows to the desired carbon fiber tows, namely, the pre-oxidation step of treating the precursor acrylonitrile fibers at a temperature within the range of 200.degree. to 300.degree. C. in an oxygen containing gas stream, the pre-carbonizing step of treating the fibers at a temperature of 700.degree. C. or less in an inert gas stream such as nitrogen gas or the like and the carbonizing step of treating the fibers at a temperature of 2000.degree. C. or less in an inert gas stream such as nitrogen gas, argon gas or the like. At the same time, it is also an important task to find precursor fibers which can easily afford the desired performance of carbon fibers.
However, the calcination stage for conversion of acrylic fibers to carbon fibers causes large physical and chemical changes and the causal relations between the two are still not clear and there still many unsolved problems. Thus, it is necessary to investigate the conditions to be possessed by acrylic fibers for carbon fibers and the optimum calcination process from industrial aspects.
As a result of the inventor's intensive research on methods for rapid and efficient production of carbon fiber tows from acrylic fibers, it has been confirmed that in the calcination process the pre-oxidation step which is the first stage is very important. That is, this step has the role of allowing to proceed the cyclizing reaction and crosslinking reaction of molecules which constitute the acrylic fibers to provide firm intermolecular bonds and to modify the molecular structure to one which can easily proceed to carbonizing reaction.
Hitherto, the pre-oxidation step has been carried out by heat-treating the precursor fibers at a temperature of 200 to 300.degree. C. in air, but requires a considerably long time to allow the reaction to proceed sufficiently. This is a big factor in the high price of carbon fibers.
The reactions at said pre-oxidation step, mainly cyclization reaction of nitrile group and oxidative crosslinking reaction caused by absorption of oxygen are greatly influenced by the heat-treating temperature and the progress of the reactions is accelerated with increase in the temperature. Therefore, when reduction of pre-oxidation time as much as possible and rapid calcination are aimed at, establishment of calcination techniques at higher temperatures is one of the most important task. For example, according to the inventors' research, when the pre-oxidation is carried out at 240.degree. C. in an air stream, a calcination time of 1 to 3 hours is required while when it is carried out at 270.degree. C., the calcination time can be reduced to 20 to 40 minutes. Density of fibers is gradually increased by the pre-oxidation and reaches about 1.35 to 1.40 g/cm.sup.3 at optimum.
One of the most severe difficulties in reduction of pre-oxidation time by increase of pre-oxidation temperature is that this brings about much self adhesion or fusion bonding between filaments of the single fibers during the calcination process. This phenomenon is recognized to nearly always occur with normal acrylic fibers although differing in its degree depending on compositions of the starting fibers, surface structure and the number of constituting single filaments.
Furthermore, carbon fibers obtained by carbonizing the oxidized fibers where self adhesion or fusion phenomenon has occurred are much deteriorated in mechanical properties, especially tensile strength to often cause breakage during carbonizing step. This clearly shows that the self adhesion or fusion phenomenon has a very bad effect on the properties of produced carbon fibers.
For preventing the fusion phenomenon, it has been proposed, e.g., in Japanese Patent Laid-Open Application (Kokai) No. 117724/74, U.S. Pat. No. 4,009,248 and U.S. Pat. No. 4,378,343 to apply a silicone compound to the surface of acrylic fibers. For example, dimethylsilicone appliedacrylic fibers obtained by immersing acrylic fibers in a solution of dimethylsilicone in an organic solvent such as ethyl alcohol are excellent in fusion resistance. However, use of organic solvents as medium for the treatment is industrially very disadvantageous. For this reason, ordinarily the application of a siloxane or aminosiloxane compound to fibers is carried out by once emulsifying said compound with an emulsifier to obtain an aqueous emulsion of said compound and then treating the fibers with this aqueous emulsion.
However, the inventors have found that according to the above method, the fusion phenomenon may occur depending on the amount of the emulsifier which is applied to the fibers and this markedly deteriorates the properties of produced carbon fibers. This invention is based on research to overcome this defect.